The prior art teaches that the importance of measuring air intake into an internal combustion engine for purposes of improving engine control. One type of mass fluid flow sensor includes a housing that projects into the main air intake tube of the engine and defines a bypass passage into which a small sample of intake air is diverted, for example, by a converging inlet section of the passage that is placed in opposition with the primary direction of airflow in the tube. A hot-wire resistive element disposed within the passage is used to generate a signal representative of instantaneous mass fluid flow through the passage, from which a controller calculates instantaneous mass airflow into the engine, as taught in co-pending U.S. patent application Ser. No. 10/126,810 tiled Apr. 19, 2002, now published as U.S. patent application No. 2003/0196486A1, and assigned to the assignee of the invention, the disclosure of which is hereby incorporated by reference.
The sensor housing is preferably provided with an exterior surface contour that cooperates with the relative location of the outlet section of the passage to create a low pressure area which draws air out of the bypass passage. The resulting “push-pull” configuration enhances the flow of fluid through the passage to thereby increase the sensor's signal-to-noise ratio. By way of example, in U.S. Pat. No. 5,556,340, the exterior surface contour is a wedge-shaped air deflector on the housing's leading edge immediately upstream of the outlet section of the passage.
The prior art further recognizes the importance of limiting the effect of back flow through the bypass passage on the airflow measurement. Thus, for example, the '340 patent teaches use of a U-shaped bypass passage that positions the inlet and outlet sections of the bypass passage relatively close to one another in the primary direction of air flow, to thereby reduce back flow by creating a similar pressure at the inlet and outlet sections of the passage under reverse flow conditions.
Unfortunately, the conduit turns or bends inherent to the U-shaped design generate fluidic losses as the diverted flow impinges against the outer wall of each passage bend, as well as due to turbulent flow induced along the inner wall of each passage bend, which disrupts the velocity profile of the diverted flow as it passes the hot-wire element, notwithstanding the use of a “push-pull” passage configuration. These effects, in turn, limit the signal-to-noise ratio and dynamic range that may be achieved with such sensors.